Vapor pressure measurements for hydrocarbons
Vapor pressure is the pressure exerted when a solid or liquid is in equilibrium with its own vapor. Stated another way, vapor pressure is a measure of the tendency of a substance to vaporize. Thus, vapor pressure values are useful in determinations of a liquid fuel's volatility, vapor locking tendency, engine performance, and potential for evaporation loss. Of these characteristics, the tendency of a liquid fuel to evaporate has particular relevance for refiners and fuel blenders. Fuel vapor is an important contributor to air pollution in certain areas, and the Environmental Protection Agency (EPA) has begun to enforce regulations to limit evaporative fuel losses. Violation of EPA-imposed vapor pressure limits on liquid fuels can lead to significant economic losses due to fines and product recalls.
Specifically, in June 1989 the vapor pressure of gasoline blended for summertime use was limited to 10.5 pounds per square inch (psi) in most states and 9.0 psi in certain areas. The applicability of these limits has since been broadened, and analogous limits have been set for other petroleum products such as diesel and jet fuels. To test their liquid fuel products for compliance with the limits, petroleum refiners and blenders use analyzers capable of direct individual sample or on-line determination of Reid Vapor Pressure (RVP). Because many fuels, including gasolines, diesel fuels, and jet fuels, are blends of hydrocarbons having different boiling points, the aggregate RVP of liquid mixtures must be determined.
To ensure the accuracy of such vapor pressure values, corrections to the RVP may be made to compensate for the partial pressure of dissolved air which may be present in the samples tested. It is also particularly important in testing fuel mixtures that certain measured volumes of the test apparatus be very accurately determined. This requirement is somewhat less stringent when dealing with a pure substance because the main factors determining a pure liquid's vapor pressure are the chemical composition of the liquid and its temperature. The relative amounts of a pure liquid and its vapor do not affect the equilibrium vapor pressure determination as long as some liquid is present. In contrast, the total vapor pressure of liquid mixtures with differing component vapor pressures depends on such factors as the ratio of vapor space to liquid volume in the liquid's container, as well as the temperature, chemical composition, and the amount of dissolved air in the liquid mixture. Hence, total vapor pressure determinations for fuel mixtures is a relatively exacting process.
The original method for determining the vapor pressure of liquid hydrocarbon fuels (either pure or mixtures) is reflected in the test for RVP defined by the American Society for Testing and Materials (ASTM) standard D4953. This method reduces variations in vapor pressure determinations by requiting uniform saturation of each test sample with dissolved air at a temperature between 32 and 40 degrees Fahrenheit. While this saturation step is beneficial when properly performed, error is introduced if the specified test volume of the saturated test sample is not actually drawn at a temperature within the allowed range.
For example, if a fluid test example is allowed to warm to a temperature above 40 degrees Fahrenheit (not an uncommon occurrence), the sample will expand and the fluid density will be reduced. A specified volume drawn from the warmed fluid will then contain less fluid mass than the same specified volume would contain if drawn from a test sample at a lower temperature. Variations in the mass of fluid introduced into a vapor pressure test instrument will cause errors in the vapor pressure determinations, and there is currently no procedure for correction of this error in the ASTM standard. Currently avail, able commercial instruments for vapor pressure measurements do not make such a correction. Hence, for fuel samples believed to have a RVP below 26 psi (i.e., most gasolines, diesels and jet fuels), following the ASTM method would ideally mean holding the fluid sample very close to the (chilled) temperature at which air saturation is carried out until the specified volume is drawn for injection into the test instrument.
The instrument for performing the ASTM RVP test includes a "gasoline" chamber and an "air" chamber, with the volume ratio of the air chamber to the gasoline chamber between the limits of 3.8 and 4.2 to 1. After preforming the chilling and air saturation steps, a fuel test sample is injected into the gasoline chamber. The air chamber (preheated to 100 degrees Fahrenheit) is then attached to the gasoline chamber so that there is an open path between the two chambers for fuel sample liquid and vapors (the test chamber expansion step). The combined chambers are immersed in a constant temperature bath at 100.degree. F. and periodically shaken until thermal equilibrium is achieved. The total pressure of the vapor and air above the remaining liquid sample is measured and reported as the RVP.
The Reid Method as specified in ASTM D4953, though extremely useful, involves tedious and time-consuming manual operations. Because of the need for obtaining accurate, precise, and reproducible results, the careful attention of one skilled in performing the test is required for 30 minutes or more per RVP determination. Because current regulations mandate frequent RVP measurements, it would be highly desirable: to shorten the test time for each one while maintaining (or improving) the accuracy, precision, and reproducibility of the original test method in an automatic on-line instrument.
Two additional ASTM test procedures are available to help meet these requirements, i.e., ASTM D5190 and ASTM D5191. These procedures allow rapid and precise determinations of vapor pressures that correlate well with the RVP data obtained by one skilled in conducting the ASTM D4953 test. Instruments developed to use these procedures, however, are generally capable of either laboratory or on-line use but not both. They also do not provide for a measurement indicative of the vapor-liquid ratio, which is often used in conjunction with RVP to characterize fuels.
The result of a vapor-liquid ratio test is actually the temperature at which a predetermined test volume of fluid will exert a predetermined pressure within a closed container having a predetermined volume. This ratio test temperature can then be related to the tendency of a fuel to vaporize in automobile fuel systems (causing vapor-lock). The most commonly used method for determining the vapor-liquid ratio of spark-ignition engine fuels is that of ASTM D2533-90. Unfortunately, this procedure is a lengthy wet-chemical laboratory method which requires mercury as a containing liquid, which makes the procedure hazardous. In addition, errors related to improper control of sample air saturation arid temperature may occur just as in the RVP test. It is apparent, then, that the above RVP and ratio test temperature determinations are both subject to significant errors, even when they are performed according to published guidelines. This may be due in part to the historical evolution of these tests, which originally were performed using equipment which made calibration and error correction relatively difficult.